Dielectric heating apparatus



Sept. 1, 1970 G. M. GILL 3,526,739

DIELECTRIC HEATING APPARATUS Filed Oct. 25, 1968 2 Sheets-Sheet l yum V aflwlz A Home y 5 Sept. 1, 1970 5 M. GILL 3,526,739

DIELECTRIC HEATING APPARATUS Filed Oct. 25, 1968 v 2 Sheets-Sheet 2 I nvenlor Attorneys United States Patent 3,526,739 DIELECTRIC HEATING APPARATUS Gilbert M. Gill, Knowl Hill, Reading, England, assignor to Dysona Industries Limited, Wokingham, Berkshire, England, a British company Filed Oct. 25, 1968, Ser. No. 770,497 Claims priority, application Great Britain, Oct. 27, 1967, 49,037/ 67 Int. Cl. H05b 5/00, 9/06 U.S. Cl. 219-1055 5 Claims ABSTRACT OF THE DISCLOSURE It has been known for many years that in designing dielectric heating apparatus, particularly apparatus working at very high frequencies (for example microwave ovens), it was necessary to provide safety measures to reduce the risk of exposure of the operator to the electromagnetic energy. In the case of a microwave oven there is usually a microswitch on the door which ensures that as soon as the door is removed from its closed position the microswitch operates to break the circuit to the H.T. contactor and thus cuts off the high frequency energy. Whilst this safety switch is normally adequate, from time to time there have been cases in which the opening of the oven door has not resulted in the removal of the H.T. from the oscillator so that the magnetron or other oscillator is still operating with the door open, constituting a serious risk to the operator. Some of these failures have been due to the welding of the. H.T. contactor t rminals, as a consequence of which the H.T. remains on although the door microswitch operates correctly. Over the years, a number of devices have been incorporated in microwave ovens in an attempt to reduce still further the risk to the operator, but to the best of our knowledge none has been entirely satisfactory.

According to the present invention, in dielectric heating apparatus having a generator operating at a high voltage to produce a heating field in a heating chamber, we provide a safety switching device which comprises: a terminal which, when the apparatus is in operation, carries the said high voltage; a movable conductor which is urged towards the said high voltage terminal and which is connected through an electrical connection to earth; means responsive to the closing of the door or lid to prevent the earthed contactor making electrical contact with the high voltage terminal and responsive to the opening of the door to permit the earthed conductor to move into contact with the high voltage terminal; and high voltage disconnecting means responsive to a flow of current in the said earthing connection to open a further contact to cause the removal of high voltage from the high voltage terminal. Preferably, the disconnecting means is of a self-latching kind which, once operated to remove high voltage from the said terminal, cannot be released without removing an access cover. In our preferred arrange ment, the earthed conductor is resiliently mounted on a pivoted bar and a contact of the disconnecting means is fixed to the pivoted bar. A portion of the pivoted bar is in the path of a member movable with the door or lid. When the door opens, in addition to the door switch which, in normal circumstances, removes the high voltage from the circuit, the pivoted bar moves under the influence of a spring firstly to bring the resiliently mounted contact into electrical contact with the high voltage terminal and secondly to bring the fixed contact on the pivoted bar into electrical contact with the high voltage terminal. Thus, if the normal door contact fails to operate, the high voltage terminal is earthed and immediately after the disconnecting means is energised to latch a contact in the open position, thereby removing the high voltage from the terminal and preventing repeated short circuiting of the high voltage supply to earth by subsequent opening and closing of the door with the equipment in the faulty condition. The temporary earthing of the high voltage supply does not in itself damage the equipment because a high voltage supply with a voltage regulator and using a high reactance transformer does not draw a greatly increased current from the supply mains when the high voltage terminal is short circuited in this way.

In order that the invention may be better understood, an example will now be described with reference to the accompanying drawings, in which:

FIG. 1 is an electrical circuit diagram of the safety switching apparatus;

FIG. 2 shows the mechanical arrangement of parts with the safety contacts in their open position; and

FIG. 3 shows the mechanical arrangement of the parts with the contacts in their closed position.

In FIG. 1, a contactor A, when energised, closes a contact A1 to connect a main supply across the primary winding of a transformer T. The secondary 0f the transformer is connected across a bridge rectifier R, the rectified output of which supplies a magnetron M. Door contacts D in the circuit of the contactor A open to deenergise the contactor when the door is opened. When this happens the contact A1 opens and breaks the circuit from the mains terminals to the primary of the transformer. In practice, other contacts are provided in series with the contactor A but these have been omitted in the circuit diagram of FIG. 1 since they are irrelevant to the present invention.

The safety switching circuit will first be described with reference to the circuit diagram of FIG. 1. In this figure, the anode of the magnetron is at earth potential and the line 4 connecting the magnetron cathode with the rectifier bridge R is at a high negative voltage. A high voltage terminal 5 is connected to the high voltage line 4. Contacts 6 and 7 are mechanically interconnected through an insulating body 8 and are so arranged that when the door opens they move downwards together. As the contacts 6 and 7 move downwards, the contact 6 will first reach the high voltage terminal 4 and will connect this terminal through a resistor R to earth. Continued downward movement of the body 8 results in the contact 7 making electrical connection with the contact 6. The voltage across the resistor R is then applied across a relay B. The relay B is energised and opens a contact B1 in series with the primary winding of the transformer, thereby interrupting the mains supply to the transformer. The relay B is of a kind having a mechanical latching arrangement built into the relay. The contact B1 is a heavy duty microswitch which releases when the winding B is energised and can only be reset by hand. In the example being described, the button for resetting the relay is located under a cover. Consequently, once relay B has operated, showing that the normal door switch has become faulty, an operator cannot restore high voltage to the magnetron but must call in a service engineer to rectify the fault on the door switch and then reset the contact B1. A suitable latching relay is made by Magnetic Devices Limited of Newmarket, England, under the designation Series 339.

The mechanical arrangement of these parts is shown in FIGS. 2 and 3. The contact 6 is at the end of a spring blade attached to a bar 11 pivoted at 12. The contact 7 is fixed to the end of the bar 11. The contact 6 is electrically connected through the metal side plate 13 to one end of the resistor R, the other end of which is electrically connected through a spring 14 to the chassis. The bar 11 is of insulating material.

Contact 7 is connected through a conductor 15 to the relay B, not shown in FIGS. 2 and 3.

The end of the pivoted bar remote from the contacts is in the path of a plate 16 which is continuously urged to the upper position shown in FIG. 3 by a spring (not shown). The end of the plate 16 is in the path of a portion 17 of a sliding door 18 and when the door moves to its closed position it carries the plate 16 in a downward direction to the position shown in FIG. 2.

Thus, with the door closed the contacts 5, 6 and 7 are separated and high voltage is applied across the magnetron to permit a heating operation. When the door is opened the plate 16 springs to its upper position permitting the pivoted bar to rotate clockwise, under the influence of the spring 14, to the position shown in FIG. 3, causing the contact 6 to meet the terminal 5 and thereafter the contact 7 to meet the terminal 6, the latter being still in electrical connection with the terminal 5. The results of this are described above.

The pillar 20 supporting the plate 18 is in fact attached to the core of a solenoid which is energised when it is desired to open the sliding door, the energisation of the solenoid causing the plate 16 to give the door 18 an initial impulse to start its upward movement.

I claim:

1. Dielectric heating apparatus having a getnerator operating at a high voltage to produce a heating field in a heating chamber and having a door or lid giving access to that chamber, the apparatus further including a safety switching device comprising:

a terminal which, when the apparatus is in operation,

carries the said high voltage;

a movable conductor which is urged towards the said high voltage terminal and which has a path through an electrical connection to earth;

means responsive to the closing of the door to prevent the earthed conductor making electrical contact with the said high voltage terminal and responsive to the opening of the door to permit such electrical contact;

and high voltage disconnecting means responsive to a flow of current in the said earthing connection to open a further contact to cause the removal of high voltage from the said first terminal.

2. Apparatus in accordance with claim 1, in which the high voltage disconnecting means includes a device for latching the said further contact in its open position to prevent restoration of the high voltage to the said terminal by a further closure of the door.

3. Apparatus in accordance with claim 1, in which the said earthed conductor is resiliently mounted on a pivoted bar which has a portion in the path of a member which moves as the result of opening and closing movement of the door or lid.

4. Apparatus in accordance with claim 3, in which the earthed conductor is a contact at one end of a spring blade which is mounted at its other end on the pivoted bar and in which an additional contact, forming part of the high voltage disconnecting means, is fixed to the pivoted bar, the arrangement being such that when the bar pivots, under the influence of a spring, upon opening of the door, the earthed conductor contact first meets the high voltage terminal and is resiliently maintained in contact with this terminal during continued pivotal move ment until it is reached by the said additional contact carried by the pivoted bar.

5. Dielectric heating apparatus in accordance with claim 1, in the form of a microwave oven including a sliding door which has a first projection for operating a microswitch to remove high voltage from a magnetron generator when the door is opened and having a second projection for operating the said safety switching device during the closing and opening movements of the door to prevent or permit respectively electrical contact between the earthed conductor and the said high voltage terminal.

References Cited UNITED STATES PATENTS 2,929,905 3/1960 Hahn 219-1055 3,104,304 9/1963 Sawada 219-1055 3,188,441 6/1965 Ojelid 21910.55 3,195,013 7/1965 Thal 21910.55 3,028,870 4/1962 Fay ZOO-61.8

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner US. Cl. X.R. 219-l0.67 

